Thermal stability of some hydrofluorocarbons as supercritical ORCs working fluids

Abstract

Organic Rankine Cycles (ORC) are used to generate electricity in renewable energy and industrial waste heat recovery systems. Supercritical ORCs have recently attracted much interest due to their higher thermal and exergy efficiencies. Hydrofluorocarbons are used as working fluids in subcritical ORCs and are also good choices for supercritical ORCs due to their good thermal performance. However, the thermal stability of the hydrofluorocarbons must be considered at high temperatures in supercritical ORCs. This study used the fluoride ion concentration to be the indicator of hydrofluorocarbon decomposition. A test system was designed and decomposition temperatures of HFC245fa, HFC152a, HFC134a, HFC236fa and HCFC123 were measured experimentally. The relationships between thermal stability and molecular structures of hydrofluorocarbons were then analyzed. The results show that these hydrofluorocarbons were good selections for supercritical ORCs by thermal stability. The hydrofluorocarbon decomposition characteristics and the effects of the decomposition products on the test system were also analyzed in this paper.